var ctr;
var lst = ["", "xml", "w1", "w2"];
var ids = [];
ctr = 0;
for (y = 0; y < lst.length; y++) {
    if (lst[y].substring(0, 1) === "w") {
        ids[y] = lst[y];
        ctr = ctr + 1;
    }
}
console.log([ids, ctr]);

OUTPUT: [[undefined, undefined, 'w1','w2'], 2]
EXPECTED OUTPUT: [['w1','w2'],2]

What am I doing wrong? The counter returned the number I have expected but why am i getting 2 undefined in my list? Why is this happening?

var ctr;
var lst = ["", "xml", "w1", "w2"];
var ids = [];
ctr = 0;
for (y = 0; y < lst.length; y++) {
    if (lst[y].substring(0, 1) === "w") {
        ids[y] = lst[y];
        ctr = ctr + 1;
    }
}
console.log([ids, ctr]);

OUTPUT: [[undefined, undefined, 'w1','w2'], 2]
EXPECTED OUTPUT: [['w1','w2'],2]

What am I doing wrong? The counter returned the number I have expected but why am i getting 2 undefined in my list? Why is this happening?

• javascript

• What you are actually getting is [[,,'w1','w2'], 2], i.e. there are no members at indexes 0 and 1. Console developers seem to have forgotten how to write elisions (or rely on toString when perhaps they should not).. – RobG Commented Sep 30, 2015 at 3:24

5 Answers 5

You need to use ids.push(lst[y]); instead of ids[y] = lst[y];, otherwise you will be assigning values to the ids array at random indexes - ie missing some index values.

In your code the values are assigned at indexes 2 and 3 missing indexes 0 and 1 causing the said output, it is because you are not assigning values to ids in all iterations - you are skipping some iterations based on the condition

You can just use a filter to get the items you want and maybe use the output array length to know how many items you have after filtering the original array.

var newArr = ["", "xml", "w1", "w2"].filter(function(x) {
  x.substring(0, 1) === 'w');
};

console.log([newArr, newArr.length]);    // [["w1", "w2"], 2]

var lst = ["", "xml", "w1", "w2"];

var result = lst.reduce(function (x, y) {
  if (y.substring(0, 1) === 'w') {
    x[0] || (x[0] = []);
    x[0].push(y);
    x[1]++ || (x[1] = 1);
  }
  return x;
}, []);

console.log(result) // [["w1","w2"],2]

The result will be same as required, interesting to note here i use null coalescing notation || more info about it here

Suggested by @FelixKing changing the accumulator

lst.reduce(function (x, y) {
  if (y.substring(0, 1) === 'w') {
    x[0].push(y);
    x[1]++;
  }
  return x;
}, [[], 0]);

If you want to take a different approach, using reduce will also work:

var res = lst.reduce(function(prevVal, curVal){
    if (curVal.substring(0, 1) === "w") {
        prevVal[0].push(curVal);
        prevVal[1] = prevVal[1] + 1;
    }
    return prevVal;
}, [[], 0]);

console.log(res); //--> [['w1', 'w2'], 2]

I remend it simply because avoiding for loops whenever possible makes for much more maintainable code, especially in Javascript where the vars used in the for loop condition itself are still members of the parent scope (i.e. they're accessible throughout the entire containing function, with a value of undefined given to them until the initialization value is given in the for loop).

Also, it prevents you from having to juggle index values in your head, and instead concentrate on what the loop is actually doing.

(My above example definitely could be cleaner, but it should give you an idea)

Using existing js , try substituting ctr for y inside if statement

var ctr;
var lst = ["", "xml", "w1", "w2"];
var ids = [];
ctr = 0;
for (y = 0; y < lst.length; y++) {
  if (lst[y].substring(0, 1) === "w") {
    ids[ctr] = lst[y];
    ctr = ctr + 1;
  } 
}

console.log([ids, ctr]);

alternatively , using Array.prototype.toString() , String.prototype.match()

var lst = ["", "xml", "w1", "w2"]
, ids = lst.toString().match(/w./g);
console.log(ids);